The invention relates to a process for treating waste paper of the type employing a beater for rough-pulping the waste paper, followed by a first sorting stage from which foreign matter separated out is processed further in a subsidiary circuit which incorporates a vibratory sorter for lightweight foreign matter and a hydro-cyclone for heavy foreign matter, the fibers separated out during this further processing being returned again to the treatment process.
Until now, the impurities present in waste paper have been separated out in most cases in the main stream, during the treatment process, in sequence in speck-removing devices and sorting devices. However, this results in heavy contamination of the purifying equipment. Moreover, there is the problem that the foreign matter left in suspension for increasingly long periods is gradually reduced to such a small size that its removal becomes increasingly difficult and under certain circumstances, no longer possible at all. Consequently, this has had a detrimental effect on the quality of the treated raw material.
It is therefore already known from German Pat. No. 2,514,162 to provide as the first sorting stage, a so-called lightweight material pulper, in which a large part of the foreign matter to be separated out is removed. However, so that the fiber losses remain within bounds, the mesh of the strainer in the vibratory sorter located in the subsidiary circuit has to be relatively coarse. This means that relatively large pieces of foreign matter also pass through the strainer mesh and so re-enter the treatment process, which leads to an undesirable concentration of the suspension with foreign matter and, with repeated circulation, these pieces of foreign matter can be broken down undesirably small so that finally they pass through small holes of a half-stuff strainer in the lightweight material pulper and remain in the fibrous suspension.